The Collaborative International Dictionary of English v.0.48:

copycat \cop"y*cat`\, copy cat \cop"y cat`\n.
   someone who copies the words or behavior of another.

   Syn: imitator, ape, aper.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

copycat
    n 1: someone who copies the words or behavior of another [syn:
         copycat, imitator, emulator, ape, aper]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "copycat":
   ape, conformist, copier, copyist, counterfeiter, cuckoo,
   dissembler, dissimulator, echo, echoer, echoist, faker, forger,
   hypocrite, imitator, impersonator, impostor, mime, mimer, mimic,
   mimicker, mocker, mockingbird, monkey, parrot, phony, plagiarist,
   poll-parrot, polly, polly-parrot, poseur, sheep, simulator